1. Field of the Invention
The present invention relates to a device and a method for wireless communication and a computer program wherein communication is carried out among multiple wireless stations as in wireless LAN (Local Area Network) or PAN (Personal Area Network). In particular, it relates to a device and a method for wireless communication and a computer program wherein data transmission is carried out by an ultra wideband system.
In more particular, the present invention relates to a device and a method for wireless communication and a computer program wherein highly accurate carrier tracking can be carried out. In further particular, it relates to a device and a method for wireless communication and a computer program wherein highly accurate carrier tracking is implemented by simpler RF circuitry.
2. Description of Related Art
WPAN (Wireless Personal Access Network) for near field communication, typified by UWB, is expected to be mounted in every household appliance and CE (Consumer Electronics) equipment in the future. Realization of P-to-P transmission at over 100 Mbps between pieces of CE equipment and home networking is anticipated. When the utilization of millimeter-wave band becomes widespread, short-range wireless communication at over 1 Gbps also becomes feasible. Ultrahigh-speed DANs (Device Area Network) for short ranges, involving storage devices and the like can also be implemented.
In recent times, wireless LAN systems to which the SS (Spread Spectrum) system is applied have become commercially practical. With use of spread spectrum, the C/I required for normal communication can be set to a level lower than 0 dB even if there is nearby any communication using the same frequency. That is, even if any other user's signal is detected at the same level as one user's signal, the one user can still carry out communication. In particular, in case of UWB, the occupied bandwidth is originally much wider than the required bit rate, and thus the SS communication method can be easily used.
The DS (Direct Spread) system is a type of the SS system. In this system, the following operation is performed: at the transmitting end, an information signal is multiplied by a random code sequence called PN (Pseudo Noise) code. Thus, the occupied band is spread and the signal is transmitted. At the receiving end, the received spread information signal is multiplied by PN code and thereby de-spread to reproduce the information signal.
There are two types of UWB transmission method, DS-UWB method and impulse-UWB method. In the DS-UWB method, the rate of spread of a DS information signal is enhanced to the maximum. In the impulse-UWB method, an impulse signal sequence having as very short a period as several hundred picoseconds or so is used to compose an information signal and this signal sequence is transmitted and received.
In the DS-UWB method, a spectrum can be controlled by PN code rate. However, this method has a problem. Since it may be required to operate a logic circuit at a high speed of the order of gigahertz, the power consumption is prone to be increased. The impulse-UWB method can be implemented by a combination of a pulse generator and a low-speed logic circuit; therefore, it brings an advantage of reduced consumption current. However, this method also has a problem. It is difficult to control a spectrum with a pulse generator.
With either method, high-speed data transmission can be implemented by spreading a signal over an ultra wide frequency band, for example, from 3 GHz to 10 GHz when the signal is transmitted and received. The occupied bandwidth is such a band of the order of gigahertz that the value obtained by dividing the occupied bandwidth by its center frequency (e.g., 1 GHz to 10 GHz) is substantially 1. This is an ultra wide frequency band even as compared with bandwidths commonly used in wireless LAN using the so-called W-CDMA and cdma2000 systems and the SS (Spread Spectrum) and OFDM (Orthogonal Frequency Division Multiplexing) systems.
There is proposed a communication device that transmits and receives a signal by pulse to avoid a problem of spectrum in the UWB communication system. (Refer to Patent Document 1, for example.) In this case, when a carrier wave is generated, the center frequency of transmission band is set to 7.5 GHz, bypassing 3 GHz or below, which is the FCC's specification for spectrum mask, and the 5 GHz band, which is used in existing wireless LAN systems. A baseband pulse is generated at time intervals equivalent to an integral submultiple of this frequency. The baseband pulse is formed as a rectangular wave whose length is equivalent to an integral multiple of one period of the carrier wave. Subsequently, the baseband pulse is frequency modulated by the carrier wave to generate a three-cycle pulse.
In such a case, 3 GHz or below and 5 GHz have substantially no energy from the beginning. Therefore, even in consideration of the FCC rules and existing communication systems using the 5 GHz band, the pulse waveform does not lose accuracy so much and energy is not lost so much. The ratio of bandwidth is lowered, and as a result, designing of antennas and RF circuits is significantly facilitated.
Wireless communication has a problem. The phase of a received signal is rotated under the influence of propagation path, such as multipath. For this reason, it may be required for a wireless receiver to carry out carrier tracking and correct the phase so that the optimum amount of phase rotation is obtained.
There is disclosed a phase rotation unit in which the method for carrier tracking differs between in cases where the transmission rate is higher than the upper-limit speed of an A-D converter and in cases where it is lower. (Refer to Patent Document 1, for example.)
More specific description will be given. In case of a transmission rate lower than the upper-limit operation speed of an A-D converter, the square values of sampled values are averaged to determine a phase shift. Then, operation is performed to return the phase by an amount equivalent to this phase shift. This is equivalent to that the rotation is corrected based on the estimation of propagation path. Therefore, high accuracy is achieved though there is restriction on speed.
In case of a transmission rate higher than the upper-limit speed of an A-D converter, template matching is carried out. In this operation, previously computed values shifted in phase by 45 degrees from the I-axis component and the Q-axis component of a received signal are passed through a simple comparator. An output from the comparator equivalent to a shift closest to the phase shift computed from a sampled value is selected, and it is taken as a value after carrier tracking.
However, this construction of a phase rotation unit has a problem. The computation of the square of a sampled value is involved, and there is apprehension that the scale of circuitry may be increased when it is incorporated into a digital circuit. In case the transmission rate is higher than the upper-limit speed of an A-D converter, a problem can arise when carrier tracking is carried out with an accuracy of 45 degrees. Half of electric power at the maximum can be lost after tracking, and this is inefficient.
[Patent Document 1] Japanese Unexamined Patent Publication No. 2004-159196
[Non-patent Document 1] International Standard ISO/IEC 8802-11: 1999(E) ANSI/IEEE Std 802.11, 1999 Edition, Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications
[Non-patent Document 2] ETSI Standard ETSI TS 101 761-1 V1.3.1 Broadband Radio Access Networks (BRAN); HIPERLAN Type 2; Data Link Control (DLC) Layer; Part 1: Basic Data Transport Functions
[Non-patent Document 3] ETSI TS 101 761-2 V1.3.1 Broadband Radio Access Networks (BRAN); HIPERLAN Type 2; Data Link Control (DLC) Layer; Part 2: Radio Link Control (RLC) sublayer
[Non-patent Document 4] “Ultra Wideband—A Revolutionary in Radio Transmission Giving Its First Cry,” the Nikkei Electronics, Mar. 11, 2002, pp. 55-66